Why do we wear pants? Even if you're currently dressed "blogger style", you've no doubt worn pants in the past. How did his fashion develop? Peter Turchin, an evolutionary biologist at the University of Connecticut, says that the domestication of horses led to pants:

The reasons why pants are advantageous when mounted atop a horse should be obvious, nonetheless, many cultures struggled to adapt, even when their very existences were threatened by superior, trouser-clad horseback riders.

Turchin details how the Romans eventually adopted braccae (known to you now as breeches) and documents the troubles a 3rd-century BC Chinese statesman, King Wuling, had getting his warriors to switch to pants from the traditional robes. "It is not that I have any doubt concerning the dress of the Hu," Wuling told an advisor. "I am afraid that everybody will laugh at me." Eventually, a different state, the Qin, conquered and unified China. They just so happened to be closest to the mounted barbarians and thus were early to the whole cavalry-and-pants thing.

Turchin speculates that because mounted warriors were generally men of relatively high status, the culture of pants could spread easily throughout male society.